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The first card contains an input field to receive the customer's name, and a button (created by the do tag) labelled "Ok". Entering data into the field and pressing the button triggers the go action of the button, which calls the getName function. This retrieves the name value that was entered and constructs a message emailmessage, and moves the browser on to display the next card, which prompts for the customer's email, using the message constructed from their name. At the bottom of this card are two options, one for registering and another to login. The scripts.wml file contains functions called by the WML pages: extern function getName() { var x = WMLBrowser. getVar ("name") ; WMLBrowser. setVar (" emailme s s age", "Enter your email, " + x); WMLBrowser. go ("#enterEmail ") ;
} extern function register() { / / move t o card with r e g i s t r a t i o n extern function login() { // send login data to server
dialog }
}
An alternative approach for user-interface design of m-commerce systems is to use Flash Lite, a restricted subset of the Flash multimedia design language (htt p://www, macro media, com).
8.10.4 Bluetooth
Bluetooth is a technology for short range (30 feet/10 metres) wireless communication between devices, enabling local area networks to be formed between PDAs, mobile phones, printers, etc. In contrast to infrared communication, bluetooth is not restricted to line-of-sight links, and can support one-to-many communication, wheras infrared is only one-to-one. One application of bluetooth could be for multiplayer games such as chess to be played by two people on different mobile phones, using bluetooth to communicate moves between the phones.
8.11. Summary
247
Java support for bluetooth operation is defined in the javax.bluetooth and javax.obex packages of the Java Micro Edition.
8.11 Summary We have described some of the technologies and applications of web service systems, and the two main frameworks, J2EE and .Net for the development of such systems.
8.12 Exercises I Represent the dating agency system (exercise 4 of the preceding chapter) in the J2EE architecture. 2 Draw a class diagram and design architecture for the following system which allows someone to remotely monitor their cat(s) via a set of movement sensors and webcams (Figure 8.14). The monitors are polled (sampled) at intervals set by the user, and the cams may also be periodically panned across the area they monitor. Individual sensors or camaras can be switched on or off. There are two interfaces designed as sets of web pages: a set of WML pages for viewing over a mobile phone, and a set of HTML pages for viewing at a PC. The system shows the rooms whose sensors were most recently triggered. Design the WML and XHTML interfaces of this system, identifying what pages are visible to the user and what command options they have. 3 Implement the messaging service of the dating agency system (exercise 4 of the preceding chapter) using JavaMail: the system stores users real email addresses, but does not show these to other users, instead it handles the emailing of messages between users, setting the 'from' address of a message to be the user's nickname on the system. Organise the data of the restaurant management system class diagram in Figure 8.15 into suitable entity beans.
5 (Project) Develop a web-service system for financial information, providing services via the XML SOAP protocol to other web systems/clients. The service answers requests for information about share prices on the UK stock exchange, and about major currency exchange rates.
Chapter 8.
248
Sensor
Cat
eb cam
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Chapter 9
Implementing the Model-Driven Architecture
This chapter describes in detail the concepts and terminology of the MDA, and the role of transformations and refactoring. We show how transformations can be formally represented and justified. We conclude with a survey of current MDA tools.
9.1
MDA
terminology
In this section we summarise the key concepts of the MDA.
Computation Independent Model (CIM) A CIM abstracts completely from implementation details, and uses terminology familiar to practitioners in the domain of the application - it is similar to the concept of a domain model, and can be used to assist in establishing requirements and expressing these in detail. It describes the system in terms of the domain in which it must operate. UML notations such as class diagrams and statecharts can be used to define CIMs. Only declarative constraints should be used to specify functional behaviour.
Model A model in MDA terms is a representation of a part of the function, structure and/or behaviour of a system [36]. A model must be in a formal representation, ie, a notation with a well-defined syntax and semantics. The subset of UML that we have been using satisfies these requirements (at least for class diagram and statechart notations) and all MDA models that we consider in this book will be expressed in UML, but other formal languages could be used instead, in principle. Model Transformation Conversion of one model of a system to another model of the same system. The central model transformation of MDA is the PIM to PSM transformation. Figure 9.1 shows two alternative strategies for carrying out this transformation. Diagram (a) shows the marking strategy for model 249
250
Chapter 9. Implementing the Model-Driven Architecture
(a) (b) ~
M Platform odle Independent --"~ ~-Platform Independent Model
instance of ~
PIM Metamodel
ingofelements Mapping
f
PlatformI n d e p ~ th M a r ~ instance
< %
PlatformSpecificModel
>
PSM Metamodel
PlatformSpecificModel - ' ~
Figure 9.1: PIM to PSM transformation strategies transformation: a mark may be a stereotype such as >, or some other identification of a particular intended implementation of a PIM model element. Several markings may apply to one element, eg, an entity may be marked as both persistent and secure, enforcing password-controlled access to it. Section 9.3 discusses how UML can be extended to support markings for such transformations. Diagram (b) shows an alternative approach in which the transformation is defined as a mapping of metamodels: we explore this approach in more detail in Section 9.2 below. Pervasive Services Services provided by platforms, such as directory services, transactions, security, event handling and notification, which are shared between all applications on that platform. Platform A set of technologies and functionalities which are available to any application based on that platform. An example is J2EE. Platform-Independent Model (PIM) A PIM captures all the essential information about and properties of a system, independent of particular implementations. The information could include"
9 9 9 9
The entities involved, with their attributes and operations; The relationships between entities, with multiplicities; Life-cycles of objects, expressed as statecharts; Logical static properties, expressed as constraints on classes and associations; 9 Logical behavioural properties, expressed as pre- and postcondition constraints on operations.
Static invariants can also implicitly make constraints on the dynamic behaviour of a system, for example a constraint b - a.size linking a role a and attribute
9.1. MDA terminology
251
b of the same class implies that whenever a is changed, b may have to be changed. Some assumptions may be made about the eventual implementation platform, eg, that operations such as addrole to add an object to a multi-valued role will be available, even if these may be actually implemented in terms of a relational database, as we described in Chapter 7. This is an example of PIM specification in terms of a virtual machine.
Platform-Specific Model A version of a PIM tailored to a particular implementation environment. Eg, from a PIM class E we may derive a class ECreateForm in the PSM which describes an input form on a web page for creating instances of E, for an online version of the system. The PSM does not add any new logical properties, but may add computational/algorithmic detail and transform the PIM elements to conform to restrictions of the platform (eg, removing many-many associations to enable representation of a data model in a relational database). A PSM must include all the details needed to implement a system, or alternatively must only rely on already-implemented specifications of a platform. The record of transformation of the PIM to the PSM is an important product of the process of producing the PSM, describing in particular which PSM elements were derived from what PIM elements, and this record should be retained.
Refactoring A parameterised behaviour-preserving program transformation that updates an application's design and underlying source code [49]. Reverse Engineering The process of extracting a design or specification from an implementation of a system: this is necessary if an organisation wishes to capture business rules which may be embedded in a legacy application. Round-trip Engineering Reverse-engineering followed by the normal forward engineering PIM to PSM transformation. This is used to port an application from one platform to another. Virtual Machine
A catalogue of operations/services which are independent of any particular platform but can be implemented in several platforms. Defining a PIM in terms of a virtual machine means that the work required to implement the PIM can be reduced, since the virtual machine may already have an implementation on the target platform. A generalisation of this is to define an application as a hierarchy of layers, each layer depending only on the interface and specification of its supplier layers, not their implementation. This means that, in principle, the PIM to PSM mapping of the application can also be split into separate mappings of each layer, with the interfaces of the layers being preserved by the mappings.
Chapter 9. Implementing the Model-Driven Architecture
252 9.2
Model t r a n s f o r m a t i o n s
Transformations on models are fundamental to the MDA approach: either as a means to improve a model to make it more generic and flexible for use as a PIM, or as a means to transform PIMs into PSMs. For reverse-engineering or round-trip engineering, PSM to PIM transformations - essentially ways of abstracting away from model-specific details - are also important. In this section we describe a number of useful transformations in detail, and give a framework for formally representing and reasoning about transformations.
9.2.1
Models
By a model, we mean a collection of UML model elements. A model can be regarded as an instance of the UML metaclass Subsystem, linked to objects representing the model elements of that particular model. For example, a simple model containing three classes, two attributes and two associations (Figure 9.2) can be represented as m l : Subsystem in the (meta) object diagram of Figure 9.3. A
9
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att 9 T
att" T
1
F i g u r e 9.2: Model m l Figure 9.4 shows a portion of the UML 2.0 metamodel. Instances of Attribute are actual attributes that are defined in UML class diagrams, likewise for Class, BehaviouralFeature, etc. A model element is partly defined by the values of its metafeatures: each Attribute instance has a type, which is a particular Type, for example. However, a model element also has a semantics, which defines what its value can be at any time point. For classes, this value is a set, also called the extent of the class, consisting of all those instances (objects) of the class that currently exist. Eg, the empty set is a possible extent of the User class in the property system PIM, as is a singleton set consisting of a single user. We will denote the extent of a model element by overlining its name: m 1.A is the set of objects whose class is ml.A (at a particular time). For an association
253
9.2. Model transformations
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Figure 9.4: UML 2.0 kernal metamodel
254
C h a p t e r 9.
I m p l e m e n t i n g the Model-Driven Architecture
end such as c r l , c r l maps each x : A to a set c r l ( x ) of elements of C (for a 1-multiplicity end, the extent is a map from objects of the source class to single objects of the target class). The extents att of attributes att : T are also maps, mapping from objects of the class of att to elements of T. Transformations between models can then be expressed as relationships between the corresponding metaobjects. For example, the 'introducing a superclass' transformation is applicable to ml, producing a model m2 (Figure 9.5). This can be formally expressed by the properties: AorB cr
att : T
Figure
9 . 5 : Model m2
m l.A = m2.A m l.B - m2.B ml.C - m2.C
That is, the classes are the same in both models. m2.AorB = m2.A 0 m2.B
The extent of the new class A o r B in m2 is the union of the extents of A and B in this model (at all times) so A o r B is abstract: ( m 2 . A o r B ) . i s A b s t r a c t = true
The extent of the attribute of this new class is likewise the union of the original two attributes: x: m2.A x: m2.B
=v m 2 . a t t ( z ) = m l . a t t A ( x ) ~ m2.att(x)= ml.attB(x)
Similarly for the roles:
x: m2.A
m2.cr(x)= ml.crl(x)
x:
m2.c
2.8
(x) = mI.
2(x)
9.2. Model transformations
255
This describes when (ml, m2) are a valid pair of models related by the 'introducing a superclass' transformation for general classes A, B, C with the features shown in Figure 9.2. In the following section we formalise some other transformations in the same manner.
9.2.2
Transformations
There are three general categories of transformations typically used in MDA development: 1. Quality improvements of models, eg: 9 introducing a superclass; 9 making partial roles into total. 2. Refinements, eg: 9 introducing design patterns; 9 transforming PIM types into PSM types (Integer to int, etc); 9 removing multiple inheritance; 9 removing many-many associations; 9 introducing primary and foreign keys; 9 source/target splitting on statecharts; 9 amalgamating subclasses. 3. Abstractions, eg: 9 replacing PSM types by PIM types. We will discuss the motivations behind some of these transformations, and illustrate how they can be formally expressed.
Introducing a Superclass This is used to simplify a class diagram and remove duplicate features from it, by recognising that these features are actually special cases of a feature of a superclass that should be present in the model. This is simple to apply if two classes have a common subset of attributes and/or operations, but the situation for common roles is slightly more subtle. Consider Figure 9.6, which shows an initial attempt at drawing a class diagram of an ambulance dispatch system. What is noticeable is that there are several associations with the same meaning: Ambulance_Hospital, Ambulance_Incident, etc, all have the meaning that the ambulance is present at the location concerned. Similarly for the associations from Patient to these classes. Thus the diagram can be rationalised by creating a new superclass Location and replacing the multiple associations from Ambulance and Patient to its subclasses by a pair of associations to Location (Figure 9.7).
256
Chapter 9. Implementing the Model-Driven Architecture
I
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0..1 Incident [ casualties: [0..1 criticI~t:egerlean~
H~O'; 1
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Figure 9.6: Ambulance dispatch class diagram version 0
Ambulance
0..1
Location
0~
0..1 Ambulance Station Incident casualties: Integer critical: Boolean Hospital Patient
Figure 9.7: Ambulance dispatch class diagram version 1
257
9.2. Model transformations
In general, this transformation should be applied if there are classes A, B and C, and associations A_B and A_C with the same meaning and same multiplicity c at the A end. The result of the transformation is to create a superclass BorC of B and C, and replace the two associations by a single A_BorC association with multiplicity cl U c2 at the BorC end, if cl and c2 are the corresponding multiplicities of the original associations (Figure 9.8), and those associations were exclusive (an A object could not be associated to both a B and a C - this is the case for the ambulance system, effectively). If the associations were not exclusive, then the multiplicity at the BorC end is cl + c2: the collection of sums x + y where x is in cl and y in c2. Table 9.1 shows the rules for combining single interval multiplicities. The new role c is
A
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l B
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ill
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$
Table 9.1: Rules for combining subclass role multiplicities the union of cl and c2" x" m2.A ~
m2.c(x) - m l . c l ( x ) U ml.c2(x)
Making Partial Roles into Total Figure 9.9 shows an example of this transformation. In the first model we need the condition
{}
258
Chapter 9. Implementing the Model-Driven Architecture br A
B
0..1
C cr
m
m
mm
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m
m
m
im
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Figure 9.9: Making partial roles total Then: m2.A = m l . A m2.B = m l . B m2.C = m l . C m2.BorC = m2.B [J m2. C z : m2.A ~ m2.r(x) - m , . b r ( x ) U m l . c r ( x )
For example, A could be BankAccount, B could be Person and C could be Company. The new superclass could be called AccountHolder. Removal of Many-many Associations This transformation (Figure 9.10) replaces a many-many association with a new class and two many-one associations. In this case, model ml has classes A and B, which are unchanged in the new model m2. m2 also has a new class C, defined to have the property that
c1: C & c2: C & c l . a r = c2.ar & c l . b r = c2.br ~
c1=c2
The original relation A_B is implemented by the composition of the new relations: x : A & y : ml.br(x) r
3c: C.x=
m2.ar(c) & y = m2.br(c)
Replace Inheritance by Association This transformation (Figure 9.11) is useful to remove situations of multiple inheritance from PIMs. In this case, m l and
9.2.
259
Model transformations
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Figure 9.10: Removing a many-many association
I
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Replacing an inheritance by an association
I
260
Chapter 9. Implementing the Model-Driven Architecture
m2 have the same classes A and B, but z" m2.B
~
ml.aatt(z) - m2.aatt(m2.ar(z))
and b - m2.ar(b) m
n
(where the extent B of a subclass B is considered as a subset of the extent A of its superclass A). In other words, to get the aatt value of some b 9 B in the new model, we need to navigate to b.ar and get its aatt value. Replace Associations by Foreign Keys This transformation (Figure 9.12) is applicable to any explicit many-one association between persistent classes. It assumes that primary keys already exist for the classes linked by the association. In model m l (the PIM) and model m2 (the PSM) the classes are the same, so
akey : T
akey : T
{identity}
{identity}
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bkey : S
{identity}
{identity}
akey : T
Figure 9.12: Replacing association by foreign key that ml.A = m2.A ml.B = m2.B For a : A , b : B : b: m l . b r ( a )
~=~ m2.akeyB(b) = m2.akeyA(a)
In other words, the association consists of those pairs a ~+ b such that the foreign key value of b is equal to the primary key value of a.
9.3
UML profiles
UML can be extended with 'profiles', which allow the definition of platformspecific constructs such as 'EntityJavaBean', 'compound key' or 'form'. A
Transformations for internet systems
9.4.
261
profile consists of a set of stereotypes, which are defined in the profile by their name and the (set of) UML metaclasses that they apply to. Figure 9.13 shows the profile we have been using for web systems. A solid-headed arrow from the Form Class
Explicit
Implicit
Persistent Association
Identity
Attribute
ForeignKey
Add
Operation
Remove
Create
List Edit
Get
Delete
Figure 9.13: Profile for web applications stereotype to the metaclass indicates that particular instances of the metaclass can be stereotyped (marked) by the stereotype. So, for example, both classes and associations can be marked as 'persistent'. Profiles can be used to define a PIM to PSM mapping, with the profile stereotypes attached to model elements determining how these elements are transformed.
9.4
Transformations for internet systems
The development process we described in Chapter 7 for online implementation of an information system can also be formally expressed as a transformation of a PIM into a PSM. This PIM to PSM transformation adopts the 'marking' strategy using a profile, whereby elements of the PIM are given a marking to indicate what elements should be used to implement them. The markings used are the stereotypes: 9 persistent on classes
262
Chapter 9. Implementing the Model-Driven Architecture 9 implicit and explicit on associations 9 create, delete, add, remove, list, check, get, edit on operations.
The following specific transformations are also used to convert the PIM data model into one suitable for implementation in a relational database: 9 Removing inheritance, either by aggregation of classes, or by replacing an inheritance by an association. 9 Removing many-many explicit associations. 9 Introducing primary keys for persistent classes if they do not already have an identity attribute. 9 Introducing foreign keys to implement many-one explicit associations. A PIM class C which is stereotyped as O; }
284
Chapter 10. Case Studies of Web System Development
Data is only added to the database if there have been no errors, in which case the command page is returned to the client. The servlet for creating properties is similar: import javax, servlet .http. * ; import javax, servlet. * ; public class createPropertyServlet { private Dbi dbi;
extends HttpServlet
public createPropertyServlet () {} public void init(ServletConfig throws ServletException { super.init(cfg) ; dbi = new Dbi();
cfg)
} public void doGet(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException { res. setContentType ("text/html"), PrintWriter pw = res.getWriter(); ErrorPage errorPage = new ErrorPage(); String propertyId = req.getParameter("propertyId"), int ipropertyId = 0; try { ipropertyId = Integer.parseInt(propertyId); } catch (Exception e) { errorPage.addMessage(propertyId + " is not an integer") ; } String propertyPrice = req.getParameter("propertyPrice"); int ipropertyPrice = 0; try { ipropertyPrice = Integer.parseInt(propertyPrice); } catch (Exception e) { errorPage.addMessage(propertyPrice + " is not an integer"); } String propertyType = req. getParameter ("propertyType") ; String propertyArea = req. getParameter ("propertyArea") ; String propertyAvailable = req.getParameter("propertyAvailable") ; String propertyBedrooms = req. getParameter ("propertyBedrooms") ; int ipropertyBedrooms = 0; try { ipropertyBedrooms = Integer.parseInt(propertyBedrooms); } catch (Exception e) { errorPage, addMessage(propertyBedrooms + " is not an integer") ; } if (errorPage.hasError()) { pw.println(errorPage); } else try { dbi. createProperty(ipropertyId, ipropertyPrice, propertyType, propertyArea, propertyAvailable, ipropertyBedrooms) ; CommandPage cp = new CommandPage() ; pw. print in (cp) ; } catch (Exception e)
10.4.
285
Servlets
{ e.printStackTrace () ; errorPage, addMessage ("Database e r r o r " ) , pw.println(errorPage) ; } pw. close () ;
public void
doPost(HttpServletlgequest req,
HttpServletKesponse res) throws ServletException, IOException { doGet(req,res) ; } public void destroy() { dbi.logoff() ; }
The servlets for listing all users and properties have no input parameters for the requests they handle, but execute a S E L E C T 9 query on the relevant database table, and present the result using a ...ResultPage: import import import import import
j ava. io. * ; java.util.*; javax, servlet .http. * ; javax, servlet. * ; java. sql. * ;
public class listUserServlet { private Dbi dbi; public listUserServlet()
extends HttpServlet
{}
public void init(ServletConfig throws ServletException { super, init (cfg) ; dbi = new Dbi();
cfg)
} public void doGet (HttpServletKequest req, HttpServletKesponse res) throws ServletException, IOException
{ res.setContentType("text/html"); PrintWriter pw = res.getWriter(); try { ResultSet resultSet = dbi.listUser(); UserKesultPage userresultpage = new UserResultPage(); while (resultSet .next ()) { userresultpage, addKow(resultSet) ; } pw. println (userresultpage) ; resultSet, close() ;
} catch (Exception e) { e.printStackTrace() ; }
286
Chapter 10. Case Studies of Web System Development pw. close () ;
} public void doPost (HttpServletKequest req, HttpServletKesponse res) throws ServletException, IOException { doGet(req,res) ; } public void destroy() { dbi.logoff(); }
The servlet for obtaining all matching properties that meet a user's requirements is: import import import import import
java. io.* ; java. ut il. * ; javax.servlet .http. * ; javax, servlet. * ; java. sql. * ;
public class getUsermatchesServlet { private Dbi dbi;
extends HttpServlet
public getUsermatchesServlet () {} public void init(ServletConfig throws ServletException { super, init (cfg) ; dbi = new Dbi();
cfg)
} public void doGet(HttpServletKequest req, HttpServletResponse res) throws ServletException, IOException { res. setContentType ("text/html") ; PrintWriter pw = res.getWriter(); ErrorPage errorPage = new ErrorPage(); String userld = req.getParameter("userld") ; int iuserId = 0; try { iuserld = Integer.parselnt(userId) ; } catch (Exception e) { errorPage.addMessage(userId + " is not an integer") ; } if (errorPage.hasError()) { pw.println(errorPage) ; } else try { KesultSet resultSet = dbi.getUsermatches(iuserId) ; PropertyKesultPage propertyresultpage = new PropertyKesultPage(); while (resultSet.next ()) { propertyresultpage, addKow(resultSet) ; } pw. println (propertyresultpage) ;
10.5. Database
287
resultSet, close () ; } catch (Exception e) { e.printStackTrace() ; errorPage, addMessage ("Database error") ; pw.println(errorPage) ; } pw. close () ;
public void doPost(HttpServletRequest req, HttpServletKesponse res) throws ServletException, IOException { doGet(req,res); } public void destroy() { dbi.logoff() ; }
PropertyResultPage is used to list all the matching properties found. Figure 10.9 shows the result of selecting the list property command, and Figure 10.10 the result of selecting the get matches command, for a user who wants two-bedroomed fiats under s in the central area.
Figure 10.9: Result of listProperty
10.5 Database The data repository of the system consists of a database, and a database interface class, in which the queries required by the property search application are defined as SQL prepared statements:
Chapter 10. Case Studies of Web System Development
288
F i g u r e 10.10: Result of getUsermatches
import java. sql. * ; public class Dbi { private Connection connection; private static String defaultDriver = ""; private static String defaultDb = ""; private PreparedStatement createUserStatement" private PreparedStatement createPropertyStatement ; private PreparedStatement listUserStatement ; private PreparedStatement listPropertyStatement ; private PreparedStatement getUsermatchesStatement ; public Dbi() { this(defaultDriver,defaultDb) ; } public Dbi(String driver, String db) { try { Class. forName (driver) ; connection = DriverManager.getConnection(db) ; createUserStatement = connection.prepareStatement ("INSEKT INT0 User " + "(userId, userName, userEmail, userMinprice," + "userMaxprice,userArea,userType,userBedrooms) VALUES " + ,, (?,?,?,?,?,?,?,?)")
;
createPropertySt atement =
connection.prepareStatement ("INSEKT INT0 Property " + "(propertyId, propertyPrice, propertyType, propertyArea," + "propertyAvailable,propertyBedrooms) VALUES (?, ?, ?, ?, ?, ?)") ; listUserStatement = connection.prepareStatement ("SELECT * FKOM User") ;
10.5. Database
289
listPropertySt atement = connection.prepareStatement ("SELECT * FKOM Property") ; getUsermat che sSt atement = connection.prepareStatement ("SELECT propertyId,propertyPrice," + "propertyType,propertyArea,propertyAvailable," + "propertyBedrooms FKOM User,Property " + "WHEKE (User.userMinprice = Property.propertyPrice AND " + "User.userArea = Property.propertyArea AND " + "User.userType = Property.propertyType AND " + "User.userBedrooms fee = 5~& age >= 5 => fee = 8 [
CreateCat catId: Integer age: Integer fee: Integer
ListCat listCats0
createCat0
Figure 10.12: Design class diagram of cat records system
commands.html
Create.~ / ~ createCat.html ~
catResults.html
ListCats~x ~
lList Cats/liStcatalltablecats in
c de~!l~v~ tabl,"" "
listCat.html
Figure 10.13: Interaction statechart of cat records system
10.6. 3SP architecture example: Cat database
293
Client Tier "~2 .
.
.
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.
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createCat.html
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\
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Presentation Tier
\
/ / / / /
createCat.jsp
listCat.jsp / ,
t
/
J
Business Tier I CatBean.java CatVO.java
Integration Tier Dbi.java
Figure 10.14: Architecture of cat records system createCat cre ateCat Error in data: Press Back to re-enter createCat performed
listCat.jsp obtains the current list of cat objects from the bean and formats them into a table
294
Chapter 10. Case Studies of Web System Development
opl(...): T1 op2(...): T2 br op3(...): T3
9 9
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S
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II~
B_/ opl(...): T1
S
S
S A
B opl(...): T1 op2(...): T2 op3(...): T3
Figure 11.7: Introducing an interface
Userlnterface check(uld: String, pwd: String): Boolean setPassword(uld: String, pwd: String, ..... newpwd: String)
Administrator Interface delete(uld: String) create(uId: String, check(uId,pwd) @pre = true => el~ pwd: String) check(uId,newpwd) = tru
post:
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. . . . . . . . . . . . . . . . .
i L
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PasswordDatabase
Figure 11.8: Example of interface factoring
i J
Chapter 11. Catalogue of Model Transformations
304 I I
attl: T1 att2:T2 att3:T3 att4:T4 att5:T5 att6:T6 att7:T7
I I I I I I
AAuxl attl: T1 att3:T3 att4:T4
I I I I
att2:T2 att7:T7 AAux2 1 att5:T5 att6:T6
F i g u r e 11.9:
Disaggregation
C o n d i t i o n s The helper/component objects always exist when the master object delegates operations to them. We used an example of this transformation to separate helper classes EditUserPage, LoginPage, etc, from servlets in the property system. Another case where it arises is in breaking up large web forms into several smaller forms (Figure 11.10 shows an example for a student accommodation agency). N a m e Factor out Sub-operations D e s c r i p t i o n An operation is factored into sub-operations. M o t i v a t i o n An operation may involve complex or repeated sub-computations. These can be factored into private helper operations of the same class, invoked from the operation. D i a g r a m Figure 11.11 shows a generic example where a complex expression exp is factored out into a separate operation m l. C o n d i t i o n s It should be checked that the helper operations do not already exist in the class or in other classes before they are created. This transformation, combined with 'introduce superclass', gives the template method pattern in the case that methods in two separate classes have the same remainder after their helper method code is factored out. A similar transformation introduces derived features: N a m e Introduce Derived Features D e s c r i p t i o n An expression e built from local features of a class, which reoccurs several times in a specification, is replaced by a new derived feature f of the class, plus the constraint f - e.
Chapter 11. Catalogue of Model Transformations
306
M o t i v a t i o n Complex repeated expressions lead to inefficient implementations. A derived feature representing the expression need only be recomputed when one of its defining features changes value. An example in the Scrabble system is the introduction of rackSize in place of rackLetters.size. N a m e Introduce Superstate D e s c r i p t i o n If states Sl, . . . , 8n of a statechart all have a common set of outgoing transitions, ie, for a non-empty set al, ..., (~m of events they have transitions tsl,~l, ---, ts~,~, etc, such that, for a given j, the tsi,~ j all have the same guards, actions and target states, then introduce a new superstate s of the si, and replace the t~i,~j by new transitions t~,~j from s to the common target of the ts,,~j, and with the same guard and actions. Common invariants of the substates can be placed on the superstate. M o t i v a t i o n This reduces the complexity of the diagram (the number of transitions is reduced by ( n - 1) 9 m) and may identify a conceptually significant state that was omitted from the original model. D i a g r a m Figure 11.12 shows this transformation. a l l actl
$2
~
T1
/
"-I
T2
~
~
r
$3 O
O
O
a
T1
Figure 11.12: Introduce superstate C o n d i t i o n s The new state can only be introduced if it does not overlap with other states (except those it entirely contains or is contained by). N a m e Introduce Entry or Exit Actions of a State D e s c r i p t i o n If all transitions tl, ..., tn into a state s have the same final sequence act of actions, factor these out and define the entry action of s to be act instead. Likewise, if all transitions with source s have a common initial action act', this can become the exit action of s.
11.1. Quality improvement transformations
307
M o t i v a t i o n This reduces the complexity and redundancy of the diagram. It may correspond to a more efficient implementation. D i a g r a m Figure 11.13 shows an example of this transformation.
i
i S1 "~el~lact/actllact2/ I~e3/act5/act6 1 ",,~ i S1 ~el lact " ~[ ] ~
I~e3/act61
$3 ~
$5 ll~..... i 7 entrS:3 ~ $5 ~ ~ actl/act2 [ ~ l exit: act5 ~~ iS2~e2/act3/act4/actl/act2~5/act7/act8 ~~ $2 1~ ~e2/act3/act4 ~7/act8
I 61
L 61
Figure 11.13: Introducing entry and exit actions
11.1.3 Other quality enhancement transformations N a m e Raise Supplier Abstraction Level D e s c r i p t i o n If class A is a client of class B, a subclass of C, make A a client of C instead if A only uses features and properties of C. M o t i v a t i o n If A depends on an over-specific class, this reduces the independence of parts of the system and makes it less easy to modify. D i a g r a m Figure 11.14 shows this transformation. C opl(...): T1
S
br opl(...): T1 op2(...): T2 op3(...): T3
"i S
opl(. .): T1
S S S
L
S S S S S
A
] opl( op2( T2 op3( .): T3
:I: T,
Figure 11.14: Raise supplier abstraction
308
Chapter 11. Catalogue of Model Transformations
C o n d i t i o n s The client must genuinely be independent of the specific features of its current supplier. N a m e Express OCL Constraints Graphically D e s c r i p t i o n Replaces textual constraints by equivalent diagram elements. M o t i v a t i o n Makes the properties of the model more immediately visually apparent, more readable and comprehensible by a wider range of people and analysis tools, and more concise. Examples are multiplicity constraints on association ends, cardinality constraints on classes, subset constraints between associations, etc. If a particular kind of constraint is needed frequently in a domain or system, then a new UML stereotype abbreviating the constraint can be introduced by means of a profile, and used in the diagram instead of the constraint formula. The 'identity' stereotype of attributes is an example of this approach. UML allows new graphical icons or display styles to be used to indicate stereotyping, in addition (eg, the identity attributes could be written in bold font, instead of labelling them with '{ identity}'). N a m e Make Partial Roles Total D e s c r i p t i o n A 0..1 multiplicity role of a class A may be turned into a 1 multiplicity role by either moving the role to a superclass of its current target, or by moving the other end to a subclass of A on which the association is total. M o t i v a t i o n Total associations are generally easier to implement and manage than partial associations. D i a g r a m Figure 11.15 shows the 'generalise target' version of this transformation. Figure 11.16 the 'specialise source' version. C o n d i t i o n s In the first version we need the condition
{} N a m e Introduce Module D e s c r i p t i o n Groups together related classes into a single unit (eg, a UML package or subsystem). M o t i v a t i o n Improves modularity of model. D i a g r a m Figure 11.17 shows this transformation. C o n d i t i o n s The classes must represent a coherent unit. Normally if a class is included in a module so are its subclasses, aggregate part classes, and those classes which are suppliers to it and not suppliers to any other class (ie, subordinate/auxiliary classes). This transformation can also be used to group data classes into EJBs.
11.1. Quality improvement transformations
309
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11.16:
-
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-
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-
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-
-
-
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-
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310
Chapter 11. Catalogue of Model Transformations Move
letterMoves
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addLetterMove(m: Move, Im: LetterMove) validateMove(m: Move, b: Board) calculateScore(m: Move, b: Board) : Integer
Figure 11.17: Introducing modules Name
Simplify Postconditions
Description A number of logical transformations can be made on postconditions of an operation to simplify its specification. Two postconditions with the forms A ~ B, A ==> C can be combined into a single postcondition A ~ B & C. Two postconditions with the forms A =~ C, B =~ C can be combined i n t o A or B =~ C. If the left hand side P of a postcondition P ~ Q is a test on the prestate, and is implied by a precondition of the operation, then the postcondition can be simplified to Q. M o t i v a t i o n It is important to make postconditions as clear and simple as possible, to improve analysability and readability.
11.2 Elaborations These transformations enrich a model with additional information. This includes obvious transformations such as adding attributes and operations to a class, adding new classes, adding new subclasses of a class (valid if the class is not a leaf class), adding constraints to a class, operation, state, association, etc. It also includes making constraints more precise, such as restricting a multiplicity range of a role, or restricting the type of an attribute. These changes may introduce inconsistency into a model, so care is needed in their application. Name
Reduce Role Multiplicity Range
Description A * or a..b multiplicity role of a class A to class B may be reduced to a 0..p or a..min(b, p) role, if we know from domain or other knowledge that p is the maximum possible number of B objects that the
11.3. Ret~nements
311
association can link to one A object. In particular, if B has cardinality p. M o t i v a t i o n Reducing multiplicities reduces the data storage requirements of the system and may enable the use of simpler data structures (eg, arrays instead of lists). It also improves the possibility of verification via tools such as model checkers [25]. D i a g r a m Figure 11.18 shows this transformation.
9I ~ br
O..p
Bp
Figure 11.18" Reducing multiplicities
11.3 Refinements These transformations are used to transform a P IM to a PSM, or to make the PIM more easy to transform to a PSM. Name
Removal of Many-many Associations
Description This transformation replaces a many-many association with a new class and two many-one associations. M o t i v a t i o n Explicit many-many associations cannot be implemented using foreign keys in a relational database - an intermediary table would need to be used instead. This transformation is the object-oriented equivalent of introducing such a table. D i a g r a m This is shown in Figure 11.19. C o n d i t i o n s The new class must link exactly those objects that were connected by the original association, and must not duplicate such links:
c1" C & c2" C & c l . a r Name
c2.ar & c l . b r - c2.br ~
cl-c2
Removal of Association Classes
Description This transformation replaces an association class with a new class and two associations. M o t i v a t i o n Association classes cannot be represented directly in many objectoriented programming languages. D i a g r a m This is shown in Figure 11.20.
312
Chapter 11. Catalogue of Model Transformations
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1 1 . 1 9 : Removing a many-many association
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11.3. Refinements
313
C o n d i t i o n s The new class must link exactly those objects that were connected by the original association, and must not duplicate such links:
cl : A_B ~: c 2 : A _ B & cl.a = c2.a & c l . b = c2.b ~
cl-c2
N a m e Weakening preconditions or strengthening postconditions D e s c r i p t i o n An operation precondition can be weakened (so that it is able to be applied in more situations without error) and/or its postcondition strengthened (so that its effect is determined more precisely). Both potentially move the method closer to implementation. D i a g r a m Figure 11.21 shows a general situation.
F i g u r e 11.21"
Weakening preconditions/strengtheningpostconditions
It should be borne in mind that the precondition of the operation in any interface used by a client will be the only precondition that such a client can assume. Working to a more general (weaker) precondition in a subclass is an issue of defensive programming: improving the robustness of the actual implementation of the operation so that even if the client does call it in an apparently invalid state (according to the interface), the operation will still have defined behaviour. N a m e Replace Inheritance by Association D e s c r i p t i o n This transformation replaces inheritance of one class on another by an association from the subclass to the superclass. M o t i v a t i o n This is useful to remove situations of multiple inheritance from PIMs, eg, for transformation to a Java-specific PSM, or for construction of a relational database schema. D i a g r a m This is shown in Figure 11.22. N a m e Replace Association by Attribute D e s c r i p t i o n In UML 2.0, attributes of a class are equivalent to certain forms of association from the class. Embedding an association into a class as an attribute can be used as a step towards implementing it as an attribute of the class.
Chapter 11. Catalogue of Model Transformations
314
I A
l B
o,
A ar
I I
F i g u r e 11.22: Replacing an inheritance by an association
t B A br: B[m]
Figure 11.23: Equivalence of attributes and associations
11.3. Refinements
315
Diagram Figure 11.23 shows the equivalence of associations and attributes. Multiplicities, visibilities, the {ordered} and {unique} constraints, and the indication / that the feature is derived, can all be mapped from the association notation into identical attribute notation. C o n d i t i o n s Qualified associations cannot be represented as attributes, as there is no corresponding attribute notation, ie, for specifying map data structures. In addition the multiplicity at the owning entity end of an association cannot be generally expressed in the attribute version. For an ordinary attribute this multiplicity is 9 and for an identity attribute it is 0..1. N a m e Introduce Primary Key D e s c r i p t i o n This transformation applies to any persistent class. If the class does not already have a primary key, it introduces a new identity attribute, of integer type, for this class, together with extensions of the constructor of the class, and a new get method, to allow initialisation and read access of this attribute. M o t i v a t i o n This is an essential step for implementation of a data model in a relational database. D i a g r a m This is shown in Figure 11.24.
attl: T1 att2:T2
I
I
akey: InAeger {identity} I attl: T1 [ att2:T2 getakey0: Integer I
I
F i g u r e 11.24: Introduce primary key
N a m e Replace Associations by Foreign Keys D e s c r i p t i o n This transformation applies to any explicit many-one association between persistent classes. It assumes that primary keys already exist for the classes linked by the association. It replaces the association by embedding values of the key of the entity at the 'one' end of the association into the entity at the 'many' end. M o t i v a t i o n This is an essential step for implementation of a data model in a relational database. D i a g r a m This is shown in Figure 11.25. C o n d i t i o n s b.akey is equal to a.akey exactly when a ~ b is in the original association. This correspondence must be maintained by implementing addbr and removebr operations in terms of the foreign key values.
316
Chapter 11. Catalogue of Model Transformations
akey : T {identity}
akey : T {identity}
I br
~
B
bkey : S {identity} akey : T
bkey : S {identity}
Figure 11.25: Replacing association by foreign key N a m e Amalgamate Subclasses into Superclass D e s c r i p t i o n This transformation is the opposite of good object-oriented design, it amalgamates all subclass features into one class, and includes a flag to indicate which class the current object really belongs to. M o t i v a t i o n This is one strategy for representing a class hierarchy in a relational database. D i a g r a m This is shown in Figure 11.26. A atta : T1
C
B
~
attb: T2
attc:
T3
*
AType isA isB isC h~
I flag = isB
"
atta :A1 l T2 | ' ' . T3 / """ flag: AType/ Iflag = isC => InvC [k *~ Iflag = isC => ay.size =
1 attb: attc:
11
*
= isC " """ "k /
a0~aY-~"~Y I
Figure 11.26: Amalgamation of subclasses transformation
11.3. Refinements
317
N a m e Make Association into Index D e s c r i p t i o n This replaces an association i that identifies a member of another role set to the same class, by an integer index. M o t i v a t i o n The resulting data structures are simpler and more efficient to implement. D i a g r a m Figure 11.27 shows this transformation where i in the first model is implemented by br[index] in the second. br'size > 0 ~>br~ ~ A index: Integer
" ~ index >= 0 & / index
/
1
1
/
/
r
/
/
/
1
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~[
1
1
B1
B2
e[G2]/act
Figure 11.28: Target splitting
e[G]/act ::>
/ /
/
/
/
/
r
/
/
A1
A2
~
e[G]/act
Figure 11.29: Source splitting
/
/
/
/
11.4.
Design patterns
319
then the precondition of ~ is augmented with the condition ( s t a t e C = sa~ & G a l ) or ... or ( s t a t e C = s a ~ , & G a n , )
and the postcondition is augmented by conjuncts (stateC@pre = s a i ~: Go~i@pre) =} s t a t e C = po~i
for i = 1, ..., nc~. M o t i v a t i o n The encoding of the statechart as explicit data and updates to this can facilitate the generation of executable code to ensure that objects of the class obey the dynamic behaviour it describes. N a m e Flattening a Statechart D e s c r i p t i o n This transformation removes composite states and expresses their semantics in terms of their substates instead: a transition from a composite state boundary becomes duplicated as a transition from each of the enclosed states (if they do not already have a transition for that event). A transition to the composite state boundary becomes a transition to its default initial state. M o t i v a t i o n The transformation reduces the complexity of the constructs used to express dynamic behaviour, making this behaviour easier to verify, although the size of the model will be increased. D i a g r a m Figure 11.30 shows a typical case of elimination of a composite state, Figure 11.31 shows the elimination of a concurrent composite state. Note that transitions for the same event in two components of the same concurrent composite state must synchronise (this is a semantic variation point in UML - semantics in which transitions for the same event can also occur independently are also possible).
11.4 Design patterns Many design patterns can be viewed as model transformations, either as refinements, or as model enhancements. For example, the builder pattern can be introduced as a segregation of construction code from other parts of a class, according to the 'disaggregation' transformation. The template method pattern can be introduced by the 'factor operations' and 'introduce superclass' patterns, as described above. N a m e Introduce State P a t t e r n D e s c r i p t i o n An operation may consist of many cases, one for each element of an enumerated type. This explicit conditional choice can be made implicit by using polymorphism and separating the behaviour for each case into a separate subclass. D i a g r a m Figure 11.32 shows a general situation. C o n d i t i o n Membership of the subclass for a state (eg, A1 for state1) in the refined model should coincide with the state attribute having that state as its value (att = state1) in the original model.
320
Chapter 11. Catalogue of Model Transformations
A
A,~~. ~e~1 (A3
1j
B
e
Cl
A2
e3 el
e5
F i g u r e 11.30: Eliminating a composite state
I
A1 1
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F i g u r e 11.31: Flattening a concurrent composite state
L
/
1
321
11.4. Design patterns
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Figure 11.32: Introducing the state pattern
A2 op(x: S) post: Pred2
Bibliography
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[41] W Royce. Managing the development of large software systems. In Proceedings of IEEE WESCON. IEEE Press, 1970. [42] J Rumbaugh, M Blaha, W Lorensen, F Eddy, and W Premerlani. Modeling and Design. Prentice Hall, 1994.
Object-Oriented
[43] The Rational Unified Process, http://www.rational.com/products/rup/, 2002. [44] Java servlet technology, http://www.java.sun.com/servlets/, Sun Microsystems, 2004. [45] The user interface hall of shame, http://www.iarchitect.com/shame.htm. [46] B Sheppard. World-championship-caliber scrabble. Artificial Intelligence, (134): 241275, 2002. [47] D Smith and C Green. Toward practical applications of software synthesis. In FMSP '96, Formal Methods in Software Practice, pages 31-39, 1996. [48] http://java.sun.com/webservices/. [49] L Tokuda and D Batory. Evolving object-oriented design with refactorings. Technical report, University of Texas. [50] J Tukey. The teaching of concrete mathematics. American Mathematical Monthly, (65): 1-9, January. [51] OMG unified modelling language specification 2.0. technology/documents / formal / uml.ht m, 2004.
OMG, http://www.omg.org/
[52] Web services architecture overview - the next stage of evolution for e-business, September 2000. www106.ibm.com/developerswork/web/library. [53] Extensible markup language (xml). http://www.w3.org/XML/. [54] Xselerator, http://www.marrowsoft.com. [55] XUI, http://xui.sourceforge.net/.
Appendix A
Scrabble History and Rules
A.I
The history of Scrabble
The original form of Scrabble was invented by American architect Alfred Mosher Butts, in the 1930s. He wanted to create a game which combined the elements of skill - in finding good moves - and chance - the random selection of letters from the bag. He based the letter frequencies on analysis of how often the different letters appeared in the New York Times cover, although he reduced the number of 'S' letters to make the game harder. His ideas were rejected by games manufacturers however, and it was not until James Brunot started to independently manufacture the game in 1948 that it began to be commercially marketed. By 1953 it had become so popular, through word of mouth and its adoption by Macy's department store, that Brunot had to license manufacture to a major American games manufacturer. The game rapidly spread in popularity throughout the English-speaking world, and more recently to other countries as well, with versions existing in most of the major languages of the world. The first Scrabble world championship took place in London in 1991.
A.2
Official Scrabble rules
These are summarised from the Scrabble guidebook. The physical contents of the game consist of: G a m e b o a r d The game is played on a board of 15 squares by 15 squares (Figure 2.22). Certain squares are designated as premium squares, and are marked as such and coloured differently on the board: ordinary squares are plain green, double letter squares are light blue, triple letter squares are dark blue, double word squares are light red, triple word squares are dark red. L e t t e r tiles There are 100 letter tiles, each marked with a letter of the alphabet (or blank) and the score of the tile. For English Scrabble, there are the following tiles (Table A.1). 325
Appendix A. Scrabble History and Rules
326
B a g This initially holds all the tiles. R a c k s Four racks, each able to hold seven tiles. Each player will have their own rack during the game, from which they select tiles to make a move. P l a y e r s Either 2, 3 or 4 players are allowed. One is nominated as the scorekeeper, and keeps track of the player's scores as the game progresses.
Letter
Score
Number of tiles
blank A
0
B
3 3 2
2 9 2 2 4 12 2 3 2 9 1 1 4 2 6 8 2 1 6 4 6 4 2 2 1 2 1
C D E F G
1
1
4 2 4
H I
1
J K L M N O P Q
8 5
R
1 1 1 1
S T U V W X Y Z
1
3 1 1
3 10
4 4 8 4 10
T a b l e A . I : Scrabble tiles (English version) A game may conveniently be divided into three stages: 1. Starting the game: The players each choose one tile from the bag. The player with the alphabetically earliest tile (with the blank preceding 'a') becomes the player with the first turn. The other players are given turn numbers in the order in which they sit, going clockwise, from the first turn player. The letters are returned to the bag, which is then shaken/shuffled. 2. Main game: Each player, starting with the first turn player, draws letters from the bag until their rack is full. The first turn player plays first, by
A.2.
.
Otticial Scrabble rules
327
placing tiles from their rack in a straight line vertically or horizontally on the board in order to form a new word or words. In the very first move at least one tile must be placed on the centre square. On subsequent moves at least one tile must be placed beside or immediately above or below a square that is already occupied by a tile. The move is checked for validity, and if valid, is accepted and its score calculated and recorded by the scorekeeper and added to that player's total score. Once the player has finished their move, the next player fills their rack and moves. Ending the game: When one player has used all the tiles on their rack and the bag is empty, the game must end. It must also end if all the players have passed twice in succession. The game can also end by agreement of all the players. The scores of the letters remaining on each player's rack are added up and subtracted from their total score. The total score of all remaining letters is added to the total score of the player with the empty rack.
Figure A.1 shows an example move midway through a game.
Figure A.I: Typical Scrabble move The validity conditions for a move are: 9 All letters played in a move must be co-linear, either vertically or horizontally, and must be placed on previously vacant squares of the board. 9 The letters played do not need to be all next to each other, however if two letters t l and t2 are played in a move, then every square between them must also be occupied by a letter by the end of the move. 9 All words formed by the move must be in the standard Scrabble word list. The words formed by the move are all maximal vertical or horizontal sequences of letters on the board, read from left to right and from top to bottom, which include at least one letter added during the move. The scoring calculation is as follows: 1. The score of a move is the sum of the scores of the words formed by the move, plus 50, if the move used all seven letters from the rack. 2. The score of an individual word is the sum of the letter scores of the letters in the move, this sum is multiplied by two for each double word score square covered in the move, and multiplied by three for each triple word score square covered in the move.
328
Appendix A. Scrabble History and Rules
3. An individual letter score is either: the score of the letter, if the letter is placed on an ordinary square, double word or triple word square, or was not placed during the move, or, if the letter was placed on a double letter square, it is twice the basic letter score, or if the letter was placed on a triple letter score its score is three times the basic letter score. Some important ancillary rules are: 1. A player can pass their turn, by not making a move, or they can replace one or more letters from their rack. In either case the turn then passes to the next player. 2. Once a letter is drawn from the bag it cannot be replaced except as part of a specific 'letter swap' move. 3. The blank tile can be used as any letter as needed, however it cannot be reused: once a letter is assigned to it in a move and it is placed on the board as part of a valid move, then it cannot be assigned a different letter in the same game. Its score is always zero. S C R A B B L E is a registered trade m a r k of J. W. Spear & Sons PLC.
Appendix B
Web Application Development Support Package
This appendix describes the Java classes HtmlPage, HtmlForm, etc, which can be used (particularly in Servlet-based systems) to generate web pages as part of an internet application. Figure B.1 shows the classes involved and their relationships. HtmlComponent
Htmlltem
HtmlText
HtmlInput
HtmlBody
HtmlHead
HtmlPage
HtmiForm
HtmlTable
/
HtmlTableData
HtmlTableRow
Figure B.I: Classes for HTML generation
HtmlBody
This describes the body of an HTML page, as a sequence of HTML A component is added to the end of the body by the add( component: HtmlComponent) method.
components.
HtmlComponent This is the superclass of all the classes describing HTML elements. It defines a collection of attribute-value pairs which can be altered by the setAttribute(att : String, val : String) method. Such pairs appear as HTML attribute settings att = val in the tag of the HTML component. The method getHtml() : String returns the ASCII text of the HTML comportent, which can then be sent to a web browser for display. HtmlForm This class describes HTML forms, as a list of HTML components within < form > and < / f o r m > tags. The method and action of the form can 329
Appendix B. Web Application Development Support Package
330
be set as attributes using setAttribute.
HtmlHead This class describes the head tag of an H T M L page. The title of the document is supplied as a string p a r a m e t e r in the constructor, eg: HtmlHead
head
= new HtmlHead("hdd
entry");
Htmllnput Describes an input element such as a text entry field or b u t t o n for use within a form. The type (such as text, password or submit) is defined using setAttribute, as are the other attributes of the element. Htmlltem This describes single-line H T M L elements such as the p a r a g r a p h marker < p > or horizontal rule < hr >. The tag is specified in the constructor, eg: HtmlItem
para
= new HtmlItem("p")
;
for a paragraph.
HtmlPage This class describes complete web pages, with a header in HtmlHead and a body in HtmlBody. The methods setHead(head : HtmIHead) and setBody(body : HtmlBody) set these elements of the page. HtmlTable This represents H T M L tables. It consists of a list of HtmITableRow instances, a new row is added at the end of the table by the method addRow( row : HtmlTableRow). HtmlTableData This represents a cell in an H T M L table, most commonly, a single piece of text. In the latter case the text is supplied as a p a r a m e t e r of the constructor: HtmlTableData(text:String). HtmlTableRow This represents a row of an H T M L table. It consists of a list of cells which are HtmlTableData instances. A new cell is added to the end of the row by the method addCell(cell : HtmlTableData). HtmlText This describes text components. The text string and the tag are supplied in the constructor HtmlText(text: String, tag : String). For example: HtmlText
nameLabel
= new HtmlText("Name:","strong")
;
generates the text "Name:" in boldface font. Other qualifiers, such as font size and colour, can be defined using setAttribute.
Appendix
C
Using Standard OCL
The constraint language LOCA that we have used here is an alternative concrete syntax for a subset of the OCL 2.0 specification notation. In LOCA we only use the OCL types OclAny (the supertype of booleans, numerics and strings), Boolean, Integer, Real and String, and the parameterised types Collection < T >, Set < T > and Sequence < T > (Figure C.1). Integer is a true subtype of Real since all operations of reals are applicable to integers and have the same results on integers. Collection
I Set
OclAny
I Sequence
I
I
Real
Boolean
I String
Integer
F i g u r e C . I : LOCA type hierarchy Table C.1 shows the mapping of LOCA syntax to normal OCL syntax, e' denotes the translation of e. The translated form of numeric operators /, +, ,, - , = , >, < = on numeric expressions is identical in OCL and LOCA. Comparisons coll < z where coll is a collection of numerics, map to
coll-~forAll(y I P) where P is the translation of y < z. Likewise for other comparison operators between collections and collections, or collections and individual elements. Navigation expressions differ slightly: 331
Appendix
332
LOCA OclAny x = y x /= y Real x+y x -
i
7 This could be: (Al(x > r.y))/8
{}
This is"
sql. size = sq2. size & (i >= 1 & i O) { current = (List) ((ArrayList) sublist = (List) ((ArrayList)
l).clone(); i).clone() ;
349
D. 4. Solutions: Chapter 5 head = sublist.get(O); sublist, remove (0) ; sublistIterator = new SublistIterator(sublist);
public List getCurrent() { return current; } public boolean atEnd() { if (l.size() == O) { return true; } return (index == l.size()
- I &&
sublistIterator, atEnd ()) ;
} public void advance() { if ( l . s i z e ( ) = = O) { return; } // can't move if (sublistIterator.atEnd()) { if (index == l.size() - i) { current = new ArrayList(); return; } // can't move index++; head = sublist.get(O); current = (List) ((ArrayList) sublist).clone() ; sublist, remove (0) ; sublistIterator = new SublistIterator(sublist) ; return;
} sublistIterator,
a d v a n c e () ;
List sb = sublistIterator.getCurrent(); current = (List) ((ArrayList) sb) .clone() current, add (0, I. get (index)) ;
;
public static void main(String[] args) { List 1 = new ArrayList(); I. add("A") ; I. add("B") ; l.add("a"); l.add("b"); l.add("c"); l.add("d"); SublistIterator si = new SublistIterator(1); do { System.out.println(si.getCurrent() ) ; si. advance () ; } while (!si.atEnd());
3
A module design for the Board module could be:
MODULE Board
Appendix D. Exercise Solutions
350 MANAGES DATA: Board, Square CONSTRAINTS C5: s l : Square & s2 : Square & sl.is0ccupied() s2. is0ccupied() => s2 : sl.adjacent.closure 0PERATIONS placeMove(b: Board, m: Move)
&
// pre: { All squares under the new moves are free } // im : m.letterMoves => // b. boardSquare [Im. x, im. y] .squareLetter = {} // pre: { either board is empty, or a new move // is adjacent to an existing one } // (b.boardSquare. squareLetter = {}) or // (LM : m.letterMoves &
//
b.boardSquare[LM.x,LS, y] .adjacent.squareLetter
/= {})
/ / post: // //
Im : m.letterMoves => b. boardSquare [Im. x, ira.y] .squareLetter
//
=
Im. letter
is0ccupied(b: Board, i: 1..15, j : 1..15) : Boolean // post : // result = // b. boardSquare [i, j] .is0ccupied()
getSquare(b: Board, i: 1..15, j: 1..15): Square // post: END
result = b.boardSquare[i,j]
J The B and C classes are similar enough to suggest the creation of a common superclass, BorC, to which the r role is directed. The many-many association could also be eliminated. Figure D.18 shows the new model. 5
Figure D.19 shows an object diagram for this interface.
6 The classes A, B, C, D are amalgamated as shown in Figure D.20. 7 This is shown in Figure D.21. 8
The classes could be:
public abstract class Person { private int consumed = 0, public void consume(int units) { consumed += units, if (overLimit (consumed)) { System.out.println("You are over your limit!");
}
D.4.
Solutions: Chapter 5
r
aatt : Real
351
BorC
/~
C catt: Real
.
B
batt: Integer
, 1
*
I
B D
dr"
t
Figure D.18: Improved class diagram
Button
boardPanel: P a n e l ~
text: String enabled: Boolean
gui: GameGUI
l
board Buttons
Figure D.19: Object diagram of OXO GUI
I AType isA isB isC isD
I
A
attl: Real [att2: Boolean / att3: Integer [att4: Real flag: AType
/
I
er/= {~' =>
nag"{isC,ism ~
Figure D.20: Class diagram after amalgamation
//clearButton
A p p e n d i x D. Exercise Solutions
352
Initial
..~Image(f)/sampled=0 selectSamplePoint(x,y)/sampled++ Loadedl ]< I
J translateImage[sampl >=IV ed
e[sample>= d10]
[ T....lated~ ~
Analysing
Figure D.21: Statechart of face recognition system
public abstract boolean overLimit(int
x);
public void reset() { consumed = O; }
class Male extends Person { public boolean overLimit(int { return x > 28; }
x)
} class Female extends Person { public boolean overLimit(int { return x > 21; }
x)
}
9 There is multiple inheritance of C on A and B. Both A and C have an abstract operation but are not abstract themselves. These problems could be corrected by"
9 Either amalgamating A and B into a single class, or replacing the inheritance of A by C by a 0..1 to 1 association from C to A. 9 Making A (or the amalgamated A B class) and C abstract. I0
The view for player number p would contain the invariant
i" l..g.players[p].playerRack.rackSize rackButtons [i] . text g.players [p] .playerRack. rackLetters [i]. symbol
D.5.
Solutions: Chapter 6
353
and
i" g.players[p].playerRack.rackSize + 1..7 rackButtons[i].text - " - " & rackButtons[i], enabled - false
D.5 Solutions: Chapter 6 1
T h e s e c o u l d be:
import j ava. util. * ; public class Branch { private String name; private String sortCode; private List customers = new ArrayList();
// of AccountHolder
} import java.util.*; public class AccountHolder { private String name; private Map personalAccounts
= new HashMap();
} public abstract class Account { private int balance; private int max0verdraft ;
} public class PremiumAccount extends Account { private int interestI{ate; private int minBalance;
} public class 0rdinaryAccount extends Account
{} 2
T h e s e c o u l d be:
public interface I { public int opi(int x); } public class B { private String n; } public class A implements I { private int x; private int y; private List br = new ArrayList();
// of B
// String -> Account
354
Appendix D. Exercise Solutions
public void setx(int { x = xx; y = lO*xx;
xx)
}
3 In Java, the specification of the lottery system can be directly coded up using the Set data structure, which discards duplicate elements: import
java. util. * ;
public class Lottery { private Set lotteryNumbers = new HashSet(); // of Integer private Set guesses = new HashSet(); // of Integer private Set correct = new HashSet(); // of Integer private int score = O; // score = correct.size() public void generateNumbers() { Random rand = new Random(); while (lotteryNumbers.size() < 6) { int index = rand. nextInt(29) + I; lotteryNumbers, add (new Integer (index)) ;
} } public void addGuess(int x) { if (guesses.size() >= 6) { return; guesses, add (new Integer (x)) ;
}
} public void calculateScore() { correct = (Set) ((HashSet) lotteryNumbers).clone(); correct, retainAll (guesses) ; score = correct, size(); System.out.println("You guessed: " + guesses); System.out.println("Lottery numbers were: " + lotteryNumbers); System.out.println("correct: " + correct); System.out.println("score = " + score);
public static void main(String[] args) { Lottery loft = new Lottery(); lott. generateNumbers () ; lott. addGuess (3) ; lott.addGuess(lO); lott.addGuess(3); lott.addGuess(13); lott.addGuess(23); lott.addGuess(30); lott.addGuess(13); loft. calculateScore () ;
lott.addGuess(31);
D.5.
355
Solutions: Chapter 6
The Java code for this system is" public class Person { private String name; private Person boss; private Company employer; public void setEmployer(Company c, Person newboss) { // pre" newboss.employer == c employer = c; boss = newboss;
}
public class Company {} 5
This could be as shown in Figure D.22. D Did ]datt [Integer String
AorB AorBId aatt I batt catt I flag Integer Real Integer Real String
BDrBId Integer I Integer
Figure D.22: Relational database schema
6
This is shown in Figure D.23. B
batt: int aatt : double
C catt: double
Figure D.23: PSM for Java
7
The DTD could be-
datt: String
356
Appendix D. Exercise Solutions
date (#PCDATA)> orderline (quantity,note,product)> quantity (#PCDATA)> note (#PCDATA)> product (description,price,productFlag)> productFlag EMPTY> productFlag value (Assembled I SelfAssembly)> description (#PCDATA)> price (#PCDATA)> customer (name,address)> name (#PCDATA)> address (#PCDATA)>
An order could be represented as: 3-5-O3
4 white kitchen chair 35 l brown kitchen table 75 Blossom Willowby 13 Privet Drive, Little Whinging, Surrey
D.6
Solutions: Chapter 7
2 The heading of the page should be in an appropriate heading tag, so the structure of the page can be correctly interpreted by screen-reader software. The image tag should include an alt attribute to explain the meaning of the image to blind users. The table should have a caption and summary, and the column headings should be in th tags, to emphasise that they are headings and not data. An improved version is:
D.6. Solutions: Chapter 7
357
Furry Friends Pet Store All our pets in stock
| Name | Breed | Age | Price |
|---|---|---|---|
| Fido | daschund | 4 months | $SO |
| Loca | siamese | 6 months | $50 |
| Manuel | rat | 3 months | $20 |
| Felix | collie | l year | $S5 |
<strong>Enter integer :
